"This meeting is not in the calendar; It may have been moved and deleted"

I have been running into several users who are recieving the following error message after receiving meeting invites.
"This meeting is not in the calendar; It may have been moved and deleted"

It's sporadic and I havn't been able to track down the cause. I havn't been able to find any event logging related to calendering that I can bump up. What would be the best way to proceed to troubleshoot?

Thanks Point. I am familiar with delegates causing this issue however I'm also running into this on my own account and I have no delegates.

In the article you refrenced it mentions"
Turn on Calendar logging for executives and for other frequent users" I'm not familiar with how to do this. Is this event loggiing I can set with Exchange (running exchange 2007)? I find no refrence to calendaring event logging. Below is a copy of my powershell query results for event log data.